Bulk growth by sublimation and thin film formation by epitaxial growth on a substrate have conventionally been known as growth methods for a silicon carbide single crystal. In the case of bulk growth by sublimation, a hexagonal (6H, 4H, and the like) silicon carbide single crystal, which is a higher temperature polytype, can be grown, and a single crystal substrate composed of silicon carbide as such can be fabricated. However, a considerable number of defects (micropipes in particular) is introduced into the crystal, and complications have arisen in regard to expanding the substrate surface area.
In contrast, the doping of impurities can be more easily controlled, the wafer diameter can be expanded, and micropipes that are problematic in sublimation can be reduced when epitaxial growth on a single crystal substrate has been used. However, there is a problem with epitaxial growth in that the density of planar defects due to differences in the lattice constant between the substrate and the silicon carbide is often increased. Silicon in particular, which is commonly used as the substrate for epitaxial growth, has a large lattice mismatch with silicon carbide. Therefore, the occurrence of anti-phase boundaries (APB) and twins in the silicon carbide single crystal growth layer is enhanced. These phenomena induce a leakage-current when semiconductor devices are fabricated, and degrade the performances of semiconductor devices composed of silicon carbide.
A method for growing a silicon carbide single crystal on a (001) face of silicon single crystal substrate whose normal axis is slightly inclined from the <001> directions toward <110> directions (to which an off-angle has been introduced) has been proposed by K. Shibahara, et al. (Non-patent Document 1) as a method of effectively reducing anti-phase boundaries.
FIG. 5 shows a schematic example of a substrate 50 to which an off-angle has been introduced (hereinafter referred to as “off-substrate”). In FIG. 5, reference numeral 50 is an off-substrate, and 51 is a step (height difference) with an atomic level height. Also in FIG. 5, the surface of the paper corresponds to the (−110) plane, and the steps 51 with an atomic level height are oriented orthogonally to the surface of the paper, i.e., the [110] direction. The steps with an atomic level height are introduced in an equidistant manner in a single direction by giving the substrate a slight incline. Therefore, epitaxial growth is carried out in a step-flow mode in the vapor growth method, which is effective in reducing the propagation of planar defects toward the introduced step edges (the direction crosswise to the steps). For this reason, the anti-phase domain preferably expands in the direction parallel to the introduced steps rather than its orthogonal direction with an increase in film thickness. Therefore, the anti-phase boundaries can be effectively reduced.
However, methods that use this off-substrate have had the following problems. FIG. 6 schematically shows the presence of anti-phase boundaries for the case in which a silicon carbide single crystal layer is formed to a fixed thickness on an off-substrate composed of a silicon single crystal. The crystal orientation in FIG. 6 is the same as that in FIG. 5. In FIG. 6, reference numeral 61 is a silicon carbide film, 62 and 63 are anti-phase boundaries, 64 is an anti-phase boundary junction, θ is the off-angle from [001] axis, and φ is the interior-angle (54.7°) between Si(001) and the anti-phase boundary.
The anti-phase boundaries 63 generated on the terraces (flat portions) of the surface of the silicon substrate annihilate at the anti-phase boundaries junction 64, but the anti-phase boundary 62 generated on a mono atomic height step of the silicon substrate does not have a junction counterpart and is therefore not eliminated, as shown in FIG. 6. In other words, methods in which an off-substrate is used have problems in that the step density of the boundary between the silicon carbide and the silicon substrate is increased, anti-phase boundaries and twin bands are, generated inevitably, and the anti-phase boundaries are not completely eliminated.
In view of the above, the present applicant has proposed (see Patent Documents 1 and 2), as a method of reducing the twin bands or the anti-phase boundaries (hereinafter generically referred to as “planar defects”) within such a silicon carbide single crystal, a technique for reducing planar defects that propagate within the silicon carbide single crystal layer by epitaxially growing a silicon carbide single crystal layer on a substrate provided with undulations whose ridges are aligned in a specific direction on the surface of the silicon substrate.
Specifically, the microscopic view of the surface of undulations fabricated on a silicon single crystal substrate is one in which slopes face each other, as shown in FIG. 5. When a silicon carbide single crystal layer is deposited on the substrate, the anti-phase boundaries, which are generated at the edge of the mono atomic height steps of the surface of the silicon carbide single crystal substrate formed on the mutually facing off-slopes, are propagated so as to face each other with increasing thickness, and these finally merge and annihilate.
[Non Patent Document 1]
Applied Physics Letters, v(50), 1987, p. 1888
[Patent Document 1]
JP-A 2000-178740
[Patent Document 2]
JP-A 2003-68655